A career highlights section is a short list, often a few bullets, that can sit under your career profile. Where the profile sums your career up, each highlight is a single specific example: a particular piece of work and what came of it.
It is an optional section. It suits applicants with specific, relevant examples that would otherwise sit deep in the work history where a quick read may miss them. If you are early in a career with one genuine standout, building it into the profile can do the same job without padding out a separate section.
Choosing them: relevance to the role
It is natural to reach for the achievements you are proudest of or worked hardest on. The better question is which of your examples relate most closely to what this role needs. An achievement that meant a great deal personally can still say nothing about the vacancy at hand.
Suppose someone spent years in service delivery and their proudest work was designing a rostering system their old team still uses. Applying for a policy role, a smaller piece of work, a consultation they ran or a brief that informed a decision, may serve better, because it resembles the job being applied for.
Work it out from the vacancy. Read the position description and note the responsibilities and any selection criteria; if the advertisement is thin, several ads for similar roles will show what they have in common. Then match your examples to those needs. Where nothing matches exactly, an example with the same shape, the same kind of problem at a different scale or in a different setting, can still make the connection.
You do not need to have carried an outcome single-handedly to mention it. If your work was a necessary part of a larger result, describe your part and the result it fed, labelled accurately: "developed the tool used to...", "provided the analysis that supported...".
Writing them
A highlight needs a little more context than an ordinary bullet, because it stands alone rather than under a job title. The reader should be able to see the situation, what you did, and what came of it:
Reduced average processing time for applications from 15 days to 6 by redesigning the assessment workflow.
Specifics are what make a highlight work, and they do not have to be figures. Where the change cannot be measured, the specifics can be what was produced, who used it, or what decision it informed. Vague versions of either kind, "significantly improved processing times", "delivered major efficiency gains", are the thing to avoid.
Keep the section short and selective. Its point is selection, and a long list defeats it.
Repetition and space
A highlight will often also appear as a bullet under the relevant role. Where the resume has room, that is not usually a problem; the two placements serve a quick scan and a closer read respectively, and varying the wording between them helps. Where space is tight or the application sets a page limit, keep the fuller version in the work history and trim the duplicate.